Normal development of the cerebral cortex requires a complex series of cellular events, including specification, proliferation, migration and differentiation, to establish the proper structure and function. Mutations that disrupt these key developmental processes give rise to cortical malformations. Despite their prevalence and societal burden, our understanding of how mutations that cause cortical malformations disrupt brain development is still limited. Miller Dieker Syndrome (MDS) is a severe developmental disorder, characterized by craniofacial dysmorphisms, reduced brain size (microcephaly), nearly absent cortical folding (lissencephaly) and devastating neurological consequences such as mental retardation and intractable epilepsy. MDS is caused by large heterozygous deletions of human band 17p13.3, harboring several dozen genes, including PAFAH1B1/LIS1. Smaller deletions or mutations in PAFAH1B1 are the major cause of Isolated Lissencephaly Sequence (ILS), which exhibits less severe lissencephaly and no additional abnormalities. Analyses of Pafah1b1 mutant mice revealed defects in neuronal migration, which is considered to be the main cellular deficiency in lissencephaly. However, the roles of most of the other genes in 17p13.3 locus in cortical development or MDS pathogenesis have not been examined. In addition, due to a lack of mouse models that recapitulate the complete genetic defects or clinical manifestations of MDS, it is unknown whether induction, proliferation or differentiation of neural stem cells is also disrupted, as might be expected for the microcephaly phenotype in MDS patients. Moreover, recent work has shown critical differences between cortical development in humans and mice, whose brains are naturally lissencephalic. These limitations necessitate the use of human brain tissue and patient-derived cells to study the complex processes that have evolved in human and how they are disrupted in cortical malformations. Towards that aim, I generated induced pluripotent stem cells (iPSCs) from MDS and ILS patients. The goal of this project is to investigate the cellular and molecular mechanisms of MDS using human stem cell models in vitro and human developing cortical tissues ex vivo. During the initial phase of the research period (K99), LIS1-dependent and independent cellular deficiencies will be identified by comparing MDS and ILS phenotypes during in vitro cortical development from patient iPSCs. In addition, cell type-specific gene expression analysis will be done to identify genes besides PAFAH1B1 that are likely to impact cortical development and MDS progression. Subsequent studies (K99+R00) will focus on systematic functional validation of novel candidates using a combination of cell biological and genetic approaches in iPSCs and human tissues. The experiments proposed here will establish and characterize novel stem cell models of ILS and MDS, define the cellular basis for these disorders and elucidate how genes deleted in MDS affect brain development. This work will improve our fundamental understanding of human cortical development and MDS pathogenesis and may lead to the identification of new therapies for major classes of developmental disorders, including microcephaly and lissencephaly.